1. Field of the Invention
The present invention relates to a resin composition, in particular, a resin composition for circuit boards.
2. Description of the Prior Art
In order to meet the current trend in electronic products, i.e. smaller, thinner, ands lighter, printed circuit boards must be highly-densified to obtain higher transmission speeds, and maintain integrity of signals. This need is achieved by lowering dielectric constant (DK) and dissipation factor (Df) of printed circuit boards to improve signal transmission speed, at the same time reducing the loss of signal transmission. In the past, the major substrate material was thermoset epoxy resin, but DK, Df and moisture absorption of epoxy resin are high, which causes signal delay in high frequency transmission. This aspect associated with signal transmission speed and quality is particularly significant for future information communication products with high frequency and high speed. Therefore, a substrate material with low Df, low DK, good heat resistance and high glass transition temperature (Tg) is a key point in the related research and development.
Polyphenylene ether (PPE), often used as a substrate material, is considered to exhibit excellent electrical properties and good resistance both to acid and alkali. Hence, it is by nature a good electronic material. However, being thermoplastic in nature, PPE is not stable at high temperatures, and it is easily soluble in halide and aromatic solvents. These shortcomings must be overcome to obtain a good substrate material for PCBs, which has good resistance to heat and solvents with the original advantages of PPE unaffected. Earliest modification method is to introduce other thermosetting resin to compensate these shortcomings. However, unfavored problems, such as bad compatibility and difficulties in further processing due to differences in chemical structures and polarity are easily induced. Failing to maintain original advantages of PPE is also one of the reasons that this method cannot be accepted. For example, U.S. Pat. No. 6,051,662 discloses that adding epoxy resin to PPE improves its resistance to heat and solvents and adhesion. But Tg is not high enough (162xcx9c170xc2x0 C.), which might affect the reliability of products when used as high density interconnecting (HDI) substrates or integrated circuit (IC) carriers. As a result, improving the resistance to heat and Tg of low DK material has become one of the most important aspects for developing high frequency substrates.
In order to overcome the above problems, an object of the invention is to provide a novel resin composition for circuit boards prepared by mixing modified BMI, epoxy resin and PPE to form a varnish with good compatibility. Glass fiber cloths are then dipped in the resin composition to make prepregs, followed by lamination of the prepregs with copper foil at high temperatures to form copper foil substrates. In other words, various combinations of BMI/epoxy resin/PPE are prepared to form semi-interpenetrating polymer network (IPN) structures. Consequently, a resin composition having high Tg, lower DK, low Df and good resistance to heat and solvents is obtained for applications as printed circuit board substrates or IC carriers.
PPE used in this invention is different from that disclosed in U.S. Pat. No. 5,880,221, i.e. PPE is modified by terpene phenol novolac resin or dicyclopentadiene phenol resin to lower its molecular weight, and to further increase the crosslinking density thereby improving Tg and heat resistance.
In order to achieve the above objects, there is provided a resin composition for circuit boards, comprising: (i) 1xcx9c15 wt % of bismaleimide modified by barbituric acid (BTA) or its derivatives; and (ii) 20xcx9c50 wt % of polyphenylene ether (PPE) chain-broken with a phenol resin; wherein the phenol resin is terpene phenol resin or dicyclopentadiene phenol resin.
Amine curing agents, imidazole catalysts and inorganic particle additives can be added to the resin composition of the invention, depending on various conditions. Preferable inorganic particle additives are aluminum hydroxide, barium sulfide, sodium carbonate, silica and silicate. The goal of additives is to increase dimensional stability, thus expanding the scope of applications of the resin composition. Since the additives are able to increase dimensional stability, the coefficient of thermal expansion (CTE) is effectively lowered.
Component (i) of the resin composition can be mixed directly with epoxy resin and PPE, or it can be mixed with novolac resin, alcohol acid resin, polyester, polyimide, polyurethane, cyanate esters, vinyl and benzocyclobutene resin.
According to the invention, PPE is reacted with suitable amounts of chain-breaking agents such as terpene novolac resin or cyclopentadiene phenol resin, and a stuitable amount of initiator (peroxide). During the reaction, molecules are rearranged to form a PPE resin with lower molecular weight of 3500xcx9c8000 and higher crosslinking density. As a result, resistances both to heat and solvent are increased.
The steps involved to prepare the resin composition of the invention are as follows:
a. adding a suitable amount of epoxy resin and solvent in a BMI/BTA mixture at 100xcx9c130xc2x0 C. for 0.5xcx9c6 hrs, where the molar ratio of BMI/BTA is 3:1xcx9c20:1;
b. preparing PPE: adding a suitable amount of phenol resin, initiator (peroxide) and solvent to PPE (molecular weight 40000) and maintaining the number average molecular weight at 3500xcx9c8000;
c. adding a suitable amount of epoxy resin and curing agent to the solution obtained in step (a);
d. combining a suitable amount of PPE obtained in step (b) and the mixture obtained in step (c), followed by adding catalyst and additive at room temperature to obtain a varnish; where the solid content is between 65 and 45%, and the amount of modified BMI is 1xcx9c15 wt % of the total solid content; and
e. dipping a glass fiber cloth in the varnish obtained in step (d) to form a prepreg, followed by laminating the prepreg with a copper foil to obtain a copper foil substrate with lower DK and high Tg.
Detailed description of various components of the resin composition of the invention is as follows: 
Wherein R1 is aryl, aliphatic, cycloaliphatic or aliphatic group containing silane; examples are: N,Nxe2x80x2ethylene-bis-maleimide, N,Nxe2x80x2-butylene-bis-maleimide, N,Nxe2x80x2-hexamethylene-bis-maleimide, N,Nxe2x80x2-phenylene-bis-maleimide, N,Nxe2x80x2-4,4-diphenyl methane-bis-maleimide, N,Nxe2x80x2-4,4-diphenyl ether-bis-maleimide, N,Nxe2x80x2-4,4-diphenyl sulfone-bis-maleimide, N,Nxe2x80x2-4,4-dicyclohexylmethane-bis-maleimide, N,Nxe2x80x2-4,4-xylene-bis-maleimide, N,Nxe2x80x2-4,4-diphenyl cyclohexane-bis-maleimide; 
Wherein R2, R3 are xe2x80x94H, xe2x80x94CH3, xe2x80x94C2H5, xe2x80x94C6H5, xe2x80x94CH(CH3)2, xe2x80x94CH2CH(CH3)2, xe2x80x94CH2CH2CH(CH3)2, or 
(3) Epoxy Resin
Diglycidyl ether of bisphenol A epoxy, tetrabromo bisphenol-A diglycidyl ether epoxy, cyclo aliphatic epoxy resin, dicyclopentadiene epoxy resin, naphthalene epoxy resin, biphenyl epoxy resin, phenol novolac epoxy resin, o-cresol novalac epoxy resin; 
Wherein Q1 is independently hydrogen, primary or secondary lower alkyl, phenyl, haloalkyl, aminoalkyl, hydroxycarbonoxy or halohydrocarbonoxy; and Q2 is hydrogen, halogen, primary or secondary lower alkyl, phenyl, haloalkyl, hydroxycarbonoxy or halohydrocarbonoxy; 
(7) Curing Agent
Diamine, H2Nxe2x80x94R4xe2x80x94NH4, where R4 is aryl, aliphatic, cycloaliphatic, aliphatic group containing silane, such as: 
Where R5 to R12 are H, CH3, C2H5, C3H7, C(CH3)2 etc, (and wherein R5 to R12 cannot be hydrogen at the same time)
(8) Catalyst (Imidazole)
1-methylimidazole
1,2-dimethylimidazole
2-methylimidazole
2-heptadecylimidazole
2-undecylimidazole
2-ethyl-4-methylimidazole
According to the resin composition provided in the invention, BMI/epoxy resin/PPE are combined in different weight percentages, semi-interpenetrating polymer network (IPN) structures are thus formed. Therefore, a resin composition having satisfactory properties, such as high Tg, lower DK, low Df and good resistances both to heat and solvents is obtained. The resin composition provided not only can be used as substrate material such as dipped glass fiber cloths, but also applicable as other substrates, for instance, high frequency transmission printed circuit boards, IC substrate, such as ball grid array (BGA), chip scale/size packages (CSP), and flip chip.
The present invention will become more fully understood from the detailed description of preferred embodiments given hereinbelow, given by way of illustration only and thus not intended to be limitative of the present invention.